1. Field of Invention
This invention pertains to the calibration of magnetic tape drives.
2. Related Art and Other Considerations
In magnetic tape drives, magnetic tape is transported past a head unit whereon at least one, and usually both, of a write head and a read head are mounted. As the tape is transported past the head unit, the heads are employed to transduce information with respect to the tape. In a recording mode, the write head records tracks on the tape. Conversely, in a read or reproduction mode, tracks previously recorded on the tape are read.
Two types of magnetic tape drives are serpentine tape drives and helical scan tape drives. In serpentine tape drives, elongated tracks are recorded parallel to the direction of tape transport, e.g., along the major dimension of the tape, typically from a first end of the tape to a second end of the tape. In helical scan tape drives, the head unit is mounted on a rotating drum around which the tape is partially wrapped at a predetermined angle. In view of the geometry, helical tracks are stripes are recorded and read by the helical scan tape drive.
In magnetic tape drives, the tape is typically housed in a cartridge. In some tape drives, such as the helical scan drives, a portion of the tape is extracted from the cartridge into a tape path for operative encounter with the head unit. In other drives, upon opening of a cartridge lid or the like, a tape path is formed in which the head unit operatively encounters the tape. In either case, the tape path of the tape drive typically includes one or more tape guiding elements for properly guiding and aligning the tape as the tape is in transit toward the head unit.
A typical tape guide element 700 shown in FIG. 7A and FIG. 7B has the shape of a spool, Guide element 700 has upper and lower edge flanges 702, 704 and a rotating, barrel-like midsection 706, all concentrically positioned about a mounting pin 708. The top of mounting pin 708 is threaded for engagement with interior threads on upper edge flange 702. An expansion spring 710 is retained between a deck of the tape drive and an underside of lower edge flange 704.
Tape T is guided between upper flange 702 and lower flange 704 in the manner shown in FIG. 7A. To accommodate linear transport of tape T, guide midsection 706 rotates about bearing 712. Bearing 712 and edge flanges 702, 704 are movable along the vertical direction as depicted by arrow 720. However, the precise vertical position of guide member 700, and thus of tape T, is adjusted and retained by upper edge flange 702. By rotation of upper edge flange 702 about the threaded top end of mounting pin 708, the tape T can be set to a proper vertical height for feeding of tape T toward the head unit.
The vertical height for guide elements such as guide 700 of FIG. 7A must be calibrated for a tape drive, e.g., upon manufacture and for maintenance of the tape drive. To this end, tape drive manufacturers have long used "master alignment tapes" for the spatial adjustment of the tape guide elements of the tape path in order to locate the position of the tape relative to the head unit, e.g. the rotating drum or scanner in a helical scan recorder rotary head device. The master alignment tapes are prepared on a well-calibrated tape drive and then removed therefrom. The master alignment tape is then inserted into a tape drive to be adjusted, e.g., a just-manufactured tape drive, and read by the adjusted tape drive. From the readback signals of a master alignment tape acquired from the tested tape drive, the technician (or robot) makes the available spatial guide adjustments of the guide elements of the adjusted tape drive until a desired readback waveform is achieved (or nearly achieved).
The overall accuracy of the master alignment tape approach depends on how the master alignment tape is constructed and how it is used. In this regard, the spatial information about the relationship between the master alignment tape and the head unit is derived from the readback signal amplitudes. Any other factors causing fluctuations in readback signal amplitudes (i.e., inconsistent head-tape contact) may influence the results.
To date, tape drive manufacturers have used "single-scan" master alignment tapes. A "single-scan master alignment tape" for a helical scan drive has a series of tracks written by only one write head using the native (+1X) linear tape speed and drum RPM.
Typically, the relationship between the recorded track pattern of the single-scan master alignment tape and the tape edge is physically measured and verified on a separate metrology system (e.g. using Ferrofluid or Kerr effect optical techniques to detect the magnetic location of the recorded track relative to the reference edge of the tape (e.g. usually bottom edge).
Single-scan master alignment tapes are read only by one read head of a tape drive, even if the tape drive has a plurality of read heads. The read head which actually reads the master alignment tape is herein called the "activated" read head. Although only one read head is employed, single-scan master alignment tapes are produced in accordance with the number of heads normally utilized in the drive to be calibrated. For example, whereas FIG. 8A shows an example of a track pattern on a normal tape for a particular helical scan drive having two write heads and two read heads, FIG. 8B shows a single-scan master alignment tape for the same helical scan drive. As another example, a helical scan drive having four write heads and four read heads, and which conventionally is referenced as a D-2 525, typically transduces information to a tape having the track pattern shown in FIG. 9A, but uses a single-scan master alignment tape as shown in FIG. 9B.
As the name implies, in the "single-scan" method each track written on the master alignment tape is scanned only once by the corresponding read head of the tape drive undergoing adjustment. To eliminate read head width effects and recorded track width effects from the readback signal amplitude, the pattern of read head paths is intentionally offset so that each read head path only partially overlaps each track recorded on the master alignment tape. For the (two write head) helical scan drive which normally reads the track pattern illustrated in FIG. 8A, FIG. 8C shows the path of an activated one of its read heads over the single-scan master calibration tape of FIG. 8B. Similarly for the (four write head) helical scan drive which normally reads the track pattern illustrated in FIG. 9A, FIG. 9C shows the path of an activated one of its read heads over the single-scan master calibration tape of FIG. 9B.
Assuming that the head-tape contact is perfectly consistent throughout the read head scan, variation in the peak readback signal amplitude (during the on-tape scan) is directly related to the variation in the spatial overlap between the read head path and the recorded track of the master alignment tape. For example, FIG. 10 shows a typical voltage waveform from a read head (after peak detection) following a single-scan master alignment tape, assuming the ideal condition of a perfect single-scan master alignment tape being read by a perfectly matched tape drive with perfectly consistent head/tape contact throughout each read head scan. Typically, the voltage waveforms from many on-tape read scans are averaged together, as depicted in FIG. 11, to improve signal to noise ratio (SNR). In FIG. 11, the amplitude V represents an overlap of the read head pass over the written track assuming perfect head-to-tape contact.
Generally, the technician (or robot) adjusts the tape drive's tape guiding elements, e.g. as explained above, to minimize any variation in the peak readback signal amplitude. This necessarily implies that the read head path shape matches the recorded track shape of the master alignment tape. However, as track densities have increased, it has become more difficult to make "perfect" master alignment tapes. Therefore, since the master alignment tape has been previously measured on a separate metrology system, the technician can also be instructed to intentionally achieve a certain amount (and shape) of peak readback signal amplitude variation that would compensate for the known errors of the master alignment tape for an "electrical" compensation for the master alignment tape error can be added by the voltage measurement system). In either case, the technician works to a fixed peak readback signal amplitude/shape target that assumes that the head-tape contact is perfectly consistent though out the read head scan and that the peak readback signal is directly related to the physical overlap of read head scan over the recorded track of the master alignment tape.
If the read head-to-tape contact is not perfectly consistent through out the scan, the single-scan method will result in a misadjustment of the tape drive in order to compensate for the poor head-tape contact. For example, if poor head-tape contact results in a loss of peak readback signal amplitude only near the start of scan region, as in the case shown in FIGS. 12D and 12D-1, the drive will be adjusted so that the read head path overlaps more of the recorded track in this area to compensate for the signal loss due to poor head-tape contact. Although the desired peak readback signal amplitude/shape is achieved, the drive's spatial alignment does not match the master alignment tape since the overlap is not consistent.
What is needed therefore, and an object of the present invention, is a master alignment tape and method of using the same for accurate and efficient adjustment.